Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 9 ■ REDO AND UNDO
ops$tkyte@ORA10G> create table t
2 as
3 select *
4 from all_objects
5 order by dbms_random.random;
Table created.
ops$tkyte@ORA10G> alter table t add constraint t_pk primary key(object_id)
2 /
Table altered.
ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'T', cascade=> true );
PL/SQL procedure successfully completed.
And now we are ready to do our modifications:
ops$tkyte@ORA10G> begin
2 for x in ( select rowid rid from t )
3 loop
4 update t set object_name = lower(object_name) where rowid = x.rid;
5 commit;
6 end loop;
7 end;
8 /
Now, while that is running, we run a query in another session. This query was reading this
table T and processing each record. I spent about 1/100 of a second processing each record
before fetching the next (simulated using DBMS_LOCK.SLEEP(0.01)). I used the FIRST_ROWS hint
in the query to have it use the index we created to read the rows out of the table via the index
sorted by OBJECT_ID. Since the data was randomly inserted into the table, we would tend to
query blocks in the table rather randomly. This block ran for only a couple of seconds before
failing:
ops$tkyte@ORA10G> declare
2 cursor c is
3 select /*+ first_rows */ object_name
4 from t
5 order by object_id;
6
7 l_object_name t.object_name%type;
8 l_rowcnt number := 0;
9 begin
10 open c;
11 loop
12 fetch c into l_object_name;
13 exit when c%notfound;
14 dbms_lock.sleep( 0.01 );
15 l_rowcnt := l_rowcnt+1;
16 end loop;